This invention relates to a method and apparatus for forming seam-welded tubes by bending a strip cylindrically, and then pressing the resultant product in the circumferential direction thereof by fin pass rolls into a tube blank, and more particularly relates to a method and apparatus for forming seam-welded tubes which is suitably applied to a process for forming seam-welded tube blanks using cage rolls.
In general, a seam-welded tube based on a forming step using cage rolls is manufactured in the following manner. As shown in FIGS. 1 and 2, a strip material 1 is gradually formed into a cylindrical shape by a preforming roll unit 2, which is used in initial and intermediate forming stages, a breakdown roll unit 3, an edge forming roll unit 4 and a cage roll unit 5. This cylindrically-formed product is then subjected to a finishing step, in which it is pressed in its circumferential direction by a fin pass roll unit 6, to form an edge portion 1a thereof stably and thereby obtain a tube blank of a predetermined size. FIGS. 2A through 2G show the strip material 1 varying gradually into a cylindrical form, wherein FIG. 2A shows a cross-sectional shape of the material 1 passing through the preforming roll unit 2; FIG. 2B, a cross-sectional shape of the material 1 passing the breakdown roll unit 3; FIG. 2C, a cross-sectional shape of the material 1 passing through the edge forming roll unit 4; FIGS. 2D-2F, the cross-sectional shapes of the material 1 passing through the cage roll unit 5; and FIG. 2G, a cross-sectional shape of the material 1 passing through the fin pass roll unit 6.
In the above-mentioned forming step using cage rolls, the so-called downhill grade forming is usually done, in which the central portion 1b of the strip material 1 is lowered to a base line L as the forming progresses, as shown in FIGS. 3-5. In this forming step, a smooth bend-forming operation is carried out in which a difference between the lengths of loci along which the edge portion 1a and the central portion 1b of the strip material 1 are moved is minimized to suppress lengthwise extension of the edge portion 1a, and the edge portion 1a is supported continuously in a restricted state by a plurality of successively-arranged outside cage rolls 7 (See FIGS. 2D-2F).
In the above seam-welded tube-forming apparatus, it is necessary to adjust the positions of the outside cage rolls 7 in accordance with the size of a tube to be formed. In order to meet the requirement, an apparatus constructed as shown in FIG. 6 was developed. This apparatus comprises a common base 8 set firmly on the groundwork, stools 9 on the common base 8 each having a surface 9a of a predetermined angle of inclination, said stools 9 being disposed in an opposed relationship so as to sandwich a line along which the strip material 1 passes, guide mechanisms 10 on the inclined surfaces 9a of the stools 9, displacement mechanisms 12 adapted to be moved by driving means 11 along the guide mechanisms 10 in the directions parallel to the inclined surfaces 9a of the stools 9, and cage rolls 7 attached to the displacement mechanisms 12 via angle-adjusting means 13.
According to this seam-welded tube-forming apparatus, when the displacement mechanisms 12 are moved along the guide mechanisms 10 by actuation of the driving means 11 as shown in FIGS. 7 and 8, the cage rolls 7 can be displaced linearly along the inclined lines 10a and 10b, so that the cage rolls 7 can be brought into contact with the strip material 1 in the positions corresponding to the size thereof.
However, this advantage can be obtained only when the downhill quantity is constant (usually 1D, wherein D is the diameter of a tube to be formed) as shown in FIG. 3. When the downhill quantity is varied between 0 and 1D (See two-dot chain lines in FIG. 3), the above apparatus cannot be used. Namely, when the downhill quantity and a flower-shaped forming pattern (loci of the edge portions 1a in FIG. 5) are determined, the mechanical system as a whole is determined so that the angles of the inclined lines 10a and 10b are also determined. Therefore, when the downhill quantity is varied between 0 and 1D, in accordance with the thickness, substance, grade and size of the strip material 1, the conventional apparatus described above cannot be used practically.
Preferred embodiments of the present invention will now be described with reference to the drawings.